FP-SG-140 Drift problems

[Patekkio]

A customer of mine has a weighing system with 6 load cells. If I acquire the load cell signals with a legacy instrument, I experience no problem, but when acquiring with FP-SG-140 the tare (and the signal when the weight is stable) seems to be a matter of random. Filter attribute is set to 15 Hz, and acquired signal is further filtered. In other words if I take readings in a mechanically stable situation the signals are not steady at all.

Is it an already known problem?

thanks for help

[Aaron_G]

What sort of load cells are you using? The FP-SG-140 pulses the excitation to load cell in order to avoid self-heating of the load cell. Self heating will degrade measurement accuracy. However, the pulsed excitation is not appropriate for all types of bridge sensors.

Have you configured the input range and bridge completion setting of the module?

Regards,
Aaron

[Patekkio]

Dear Aron,
I'm using full bridge, shear type, 350 ohms strain-gage load cells, so i don't need bridge completion. Input range was correctly configured with respect to cells' full scale.
What kind of bridge sensors are no suitable for FP-SG-140?
thanks for help, ciao

giovanni

[Patekkio]

Aron,
I tried (almost) every combination of gains, supply voltage, filter settings and keep on with the same problems. I do suspect it's due to the pulsed excitation. With the same cells, I tried with a HBM DC bridge and a PENCO AC bridge. AC (1kHz) bridge works a little better. It seems than pulsed excitation is not suitable for such a use.
regards,
Giovanni

[Rainer Ehrt]

Hi Giovanni,

maybe there is a problem with the cable capacity and the SG-140 measures
on the trailing edge of the pulse?
How long is your cable, is it shielded, is the shield grounded. But I
don't really believe this is the problem.

Usually its rather a problem of the power supply or a ground loop with the
cable shielding. I had similar problems while using a switching power
supply. After I spent _each_ of the power consumers a _separate_,
traditional, heavy transformer- supply, all problems vanished.

Is there possibly a ground loop with the shielding? Since you had no probs
using the legacy instrument, i guess it was a battery powerd instrument
(potential-free).

How about using all the cabeling and connectors, as you did before, but
connect four 330 Ohm res
istors as a bridge, instead of the load cell? If
you get stable values then, the problem should be a ground loop.

Ciao,
Rainer

[Patekkio]

Hi Rainer,
thanks for yor answer. As a matter of fact I'd exclude any kind of ground loops and/or shielding problems. They're usually such a crucial issue (especially with pulsed excitation) that I took great care about them. As long as with a AC excited bridge I get a 'perfect' signal, I do think that with FP there's a combination of: transient response to step signal, an irregular self heating (although the excitation is pulsed, voltage is applied to the bridge for some tens ms), and an unstable excitation. Sometimes I had similar problems with FP RTD modules.
According to my experience (I installed more than 100 FP modules) I'd prefer to spend some more money and have a non-switched excitation!
ciao

giovanni

ps: all legacy instruments were
220V AC powered!

[Rainer Ehrt]

Hi Giovanni,


> thanks for yor answer. As a matter of fact I'd exclude any kind of
> ground loops and/or shielding problems. They're usually such a crucial
> issue (especially with pulsed excitation) that I took great care about
> them. As long as with a AC excited bridge I get a 'perfect' signal, I
> do think that with FP there's a combination of: transient response to
> step signal, an irregular self heating (although the excitation is
> pulsed, voltage is applied to the bridge for some tens ms), and an
> unstable excitation. Sometimes I had similar problems with FP RTD
> modules.

Same with me. In all the past I used AC- exited bridges and allways had
perfect signals. Maybe this is interesting for you. I connected a
oscilloscope to a channel of a SG-140, with the adjustments half-bridge
completition = on, Uexc = 5V, filter = 15 Hz, Range = +/- 3,9 mV/V. The
measured Ti = 320ms, Tp = 2250ms. I monitored this with the FP-Explorer in
default configuration- just monitorin one channel.
Vexc(Ti) = 4.7V, Vexc(Tp) = 0V; Vin+(Ti) = 2,4V. Instead of strain gauges,
I connected 2 metalfilm resistors with 120 Ohm/ 1% each as a half-bridge
to the module for testing. The signal shape itself was a perfect
pulse/pause (Ti/TP).
So I don't really think you have to worry about the shape. And the signal
voltage the FP-Explorer showed was very stable. It sometimes changed about
+/- 100nV. If you keep in mind that 16bit resolution in this input range
means a resolution of about 119nV/V, this is a very good result.
There is another thing to be aware of, if you use the SG-140 with
half-bridges. If you do so and swith on the half bridge completition, you
will notice that you won't get a 0-display if the SG-140 input is feed
with 0.000mV. The module- internal halfbridge completition has an offset
with value about +/- 1.xxx mV/V and is different for every channel and
module. I had this tested with 2 new out-of-the-box module, and one
borrowed one of NI. Maybe you've seen this a couple threads above. There
are no problems like this, if you use full- bridges.

Maybe you should give it a try, just connecting metalfilm resistors
instead of the load cell for testing purposes. If the unsteady signals
still show, the reason must be some place else, but not a matter of the
load-cell.

Good look,
and please keep me on track, what youre doing, because there aren't to
many people using the SG-140.

Rainer

[Aaron_G]

Rainer,

Please see my post to your thread. The method you are using to determine offset error in half-bridge mode is incorrect. The +/- 1 mV/V reading that you are seeing is due to the resistors and not the module. The module does not have an offset error that is 1/4 of the range.

Regards,
Aaron